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Under bed extractor fan ?

Dec 29, 2018
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My autotrail tracker has an end bed under which is the combi heating system and hot air pipework and ducting. After the heating had been on a while I happened to lift the bed up to access the storage underneath and was met with a blast of hot air. There are no leaks or anything. It's just the heat on the case of the heater and pipe ducts that builds up. I was wondering if anyone had fitted a 12v extractor fan to blow this wasted hot air into the habitation area. Thanks.
 
Sep 20, 2011
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I don’t recall ever reading of a ‘waste-heat recovery fan’ being added to a Truma Combi-based system, but you might find someone has done this if you enquire on one of the bigger motorhome forums.

Whether adding a fan would be worthwhile will depend on how Auto-Trail has installed the heating system. If the Combi heater is taking in air from the storage area beneath the bed (rather than from the living-area outside the bed where the heater should be getting its air) if you fit a fan to exhaust warm air from the storage area, you’ll be preventing the heater from ‘breathing in’ that air and blowing it around the motorhome.

As an alternative, you might consider insulating the trunks (Webasto markets “Thermoduct” sleeving) as this would reduce the heat radiating from the trunks that are beneath the bed and increase the warmth of the air that those trunks are delivering around your motorhome.
 
Dec 29, 2018
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Thanks and good points raised. Others have suggested one or two vents around the bed to allow the hot air to circulate. More thought required I think.
 
Sep 20, 2011
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40
18,585
If your motorhome’s Combi heater is drawing in air from within the storage area beneath the bed, then adding vents into that storage area will not allow escape of the heat produced by the heater itself and the warm-air trunking, as the heater will then draw air into the storage area through the vents.

However, if the Combi is drawing in most (or all!) of its air from within the under-bed storage area, rather than from the motorhome’s living-area outside the under-bed storage area, then adding vents (preferably positioned near the Combi’s air-intake fan) will - as suggested - allow the heater to perform its design-objective of recirculating warmed air around the motorhome’s living area. So, if the air in the under-bed storage area is getting very hot due to the heater primarlly being unable to obtain cooler air from outside the storage area, instalingl vents into the storage area should increase the heating system’s efficiency and might help to reduce heat build-up beneath the bed.

Truma’s advice on fitting a Combi heater is often ignored completely by motorhome converters, with the result that the standard of a Combi-based heating-system's installation generally varies from just-about-adequate to atrocious. Correcting all the design faults in a poorly-executed installation is usually impracticable, but it should be possible to make useful improvements - and ensuring that the Combi heater can ‘breathe’ efficiently is the first thing to do.

When you are using the Combi just to heat water in the summer, it will still radiate a good deal of heat into the under-bed storage area, and you are unlikely to want to extract that heat into your Tracker’s living-area - so your extractor-fan idea would not be applicable to warm weather usage. You could try adding a fan (I can’t see it doing any harm) but I suggest you address any obvious shortcomings in your motorhome’s heating installation first.
 

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